The invention relates to applying a coating for providing protection against oxidation on parts made of thermostructural composite materials that contain carbon.
Thermostructural composite materials are characterized by their mechanical properties that make them suitable for constituting structural parts, and by their ability to retain these mechanical properties at high temperatures. They are constituted by fiber reinforcement densified by a matrix of refractory material that fills the pores within the fiber reinforcement at least in part. The materials constituting the fiber reinforcement and the matrix are typically selected from carbon and ceramics. Examples of thermostructural composite materials are carbon/carbon (C/C) composites and ceramic matrix composites (CMCs) such as C/SiC (carbon fiber reinforcement and silicon carbide matrix) or C/C—SiC (carbon fiber reinforcement and a matrix both of carbon and of silicon carbide), or indeed C/C—SiC—Si (C/C composite silicided by reacting with Si).
Very frequently, thermostructural composite materials contain carbon, whether constituting fibers, constituting at least part of the matrix, or indeed constituting an interphase layer formed on the fibers to provide sufficient bonding with the matrix. Thus, it is essential to provide protection against oxidation in order to avoid rapid deterioration of parts made of such composite materials when such parts are used in an oxidizing atmosphere at a temperature higher than 350° C.
There exists abundant literature concerning the formation of anti-oxidation protective coatings for parts made at least in part out of carbon or graphite.
For parts made of thermostructural composite materials containing carbon, in particular C/C composites, it is known to form a protective coating made at least in part from a composition containing boron, and more particularly a composition having self-healing properties. The term “self-healing” is used of a composition to mean that by passing to a viscous state at the utilization temperature of the part, it serves to plug any cracks that form in the coating or the protective layer. Otherwise, in an oxidizing atmosphere, such cracks give access to the oxygen in the surroundings to reach the composite material and to infiltrate into the residual pores thereof. Commonly-used self-healing compositions are boron glasses, in particular borosilicate glasses. By way of example, reference may be made to document U.S. Pat. No. 4,613,522.
The oxide B2O3 is the essential element of boron protective compositions. It possesses a relatively low melting temperature (about 450° C.) and its presence provides adequate ability to wet the carbon surface that is to be protected. When conditions of use require the use of specific self-healing vitreous mixtures, formulated so as to possess appropriate viscosity in an intended temperature range, it is essential to combine such mixtures with B2O3 in order to form a continuous protective film on the surface of the substrate.
Depending on the conditions of use, B2O3 may volatilize, either progressively as from 500° C. (in particular in a wet atmosphere), or more rapidly at higher temperatures. Above 1100° C. volatization becomes so fast that the effectiveness of the protective mixtures present disappears as a result of losing wetting ability, even for uses of very short duration.
One way of slowing down the complete disappearance of B2O3 consists in adding metallic borides into the protective composition, which borides are capable of reforming B2O3 progressively by oxidation, as the existing B2O3 volatilizes. Document U.S. Pat. No. 5,420,084 describes a protective coating serving in particular to protect C/C composite material parts against oxidation up to 1350° C., the protective coating being formed by a mixture of zirconium diboride ZrB2 and colloidal silica SiO2.
A method is also known from document U.S. Pat. No. 6,740,408 for forming a protective coating for C/C composite material parts. That method comprises forming on the part a coating that contains a mixture of a boride powder constituted in the majority by titanium diboride TiB2, by a powder of a refractory oxide preventing healing properties by forming a silicate glass, and containing in the majority a borosilicate mixture (such as a powder of an SiO2-B2O3 mixture), and by a binder constituted by a refractory ceramic precursor resin (e.g. a resin selected from polycarbosilanes (PCS), precursors of silicon carbide SiC; and polytitanocarbosilanes (PCTS)), the precursor then being transformed into a ceramic. Titanium diboride TiB2 constitutes a B2O3 regenerator. This is because on oxidizing, progressively as from 550° C., and more quickly as from 1100° C., TiB2 compensates for the disappearance of B2O3 by generating B2O3+TiO2. The TiO2 oxide disperses in the oxides of the silicate glass and contributes to increasing viscosity while maintaining its healing power. The protective layer as obtained in this way thus provides effective and durable protection against oxidation for C/C composite parts used in a wet environment at high temperature.
Nevertheless, the effectiveness of the protection of known coatings having self-healing properties is no longer ensured at temperatures greater than about 1450° C., even for coatings that contain B2O3 regenerators such as titanium diboride TiB2 or zirconium diboride ZrB2. Above 1450° C., it is observed that the oxide B2O3 volatilizes completely, including the oxide obtained by regeneration. Under such conditions, it is no longer possible to form a continuous protective film on the surface of the part, in particular because of insufficient ability to wet carbon. The loss of the effectiveness of protection at such temperatures is even greater when the part is exposed for a long duration.
Although it is relatively easy to devise vitreous mixtures capable of softening at temperatures higher than 1450° C. in order to perform a healing role, the direct use of such mixtures is unfortunately not possible because of the absence of B2O3 at such temperature levels, thus leading to the protective system losing the ability to wet carbon.
The solution that is generally adopted under such circumstances consists in applying the protective mixture onto a surface of silicon carbide (SiC) instead of a surface of carbon. That requires an SiC primer underlayer to be formed that is obtained either by a reaction technique in which silicon is caused to react chemically with the carbon surface that is to be protected (T<1400° C., argon atmosphere), or else by chemical vapor infiltration (CVI).
The preparation of such an underlayer thus constitutes an additional step in forming the protective coating, which step is additionally highly complex, given the operating conditions involved.
There exists a need to provide protection for parts used in an oxidizing atmosphere at temperatures higher than 1450° C.
This applies in particular to the diverging portions of nozzles for hydrogen and oxygen rocket engines where the water vapor produced and ejected through the nozzle creates an environment that is wet and oxidizing.
This also applies to C/C composite brake disks of the kind used in aviation, when landing and taxiing on wet runways.